The instant invention relates to the correction of an error-prone measuring signal of a pair of scanning rollers which is used to measure the thickness of a fiber fleece or fiber sliver on a pre-spinning machine in the textile industry. The measuring signal is influenced by the circularity error of the pair of scanning rollers and/or by the eccentric mounting of the pair of scanning rollers.
The thickness of a fiber fleece or of a fiber sliver is measured by means of a pair of scanning rollers. For the sake of simplification, only a fiber sliver is mentioned hereinafter even though all statements made concerning the scanning roller also apply to measuring the thickness of a fiber fleece. A representative signal, the measuring signal, is derived from the measured thickness. The measuring signal is conveyed to a signal processing device. The signal processor acts with its output signal upon a device for the drafting of the fiber sliver or is used thereafter to monitor quality. The signal processor may thus produce a change in drafting at the precise instant when a different sliver thickness is present at the drafting point.
The pair of scanning rollers is designed so that a stationary, rotatable scanning roller is assigned opposite to the other rotatable and swivelling scanning roller. The two scanning rollers are pre-stressed by means of a spring. The swivelling scanning roller is swivelled out as a function of the thickness of the fiber sliver which is conveyed between the pair of scanning rollers. The angle of swivel is transformed into an electric signal, the measuring signal. As is known, a scanning roller pair is installed before a draw frame. A pair of scanning rollers is advantageously simple in mechanical structure, robust and therefore economical.
In the operation of the pair of scanning rollers, it may occur that the measuring signal is distorted. It is known that in a pair of scanning rollers operating as mechanical sensors, an influence is exerted upon the measuring signal by tolerances in the scanning rollers and tolerances of the scanning roller mountings. Deviations from the ideal geometry of a scanning roller body may manifest themselves in diameter differences over the length of the cylindrical body of the scanning roller. Similarly, deviations from a centered mounting of the scanning rollers also lead to eccentricity. The tolerances in the geometric dimensions of the scanning rollers are superimposed on the tolerances of the mounting of the scanning rollers. These tolerances then lead to an error in the measuring signal that must not be underestimated. It is a periodic error which manifests itself with every revolution of a scanning roller.
In the state of the art the deviations have been accepted in the past, and the utilization of the scanning rollers in measuring the fiber sliver thickness was therefore limited to applications where the influence of errors appeared to be acceptable. Although it is reliable and robust in practical use, it was not possible, for the above-mentioned reasons, to use the scanning roller in general application to measure the fiber sliver thickness on pre-spinning machines.
If a pair of scanning rollers is used as a measuring element only in order to determine the long-term fluctuations of the fiber sliver, the consequences of these errors remain comparatively minor ones. The error is also a minor one if the fiber sliver thickness to be measured is relatively great. This is the case, for example, with a doubled fiber sliver before it is drafted in the draw frame.
When a scanning roller is used in determining measuring signals for the control of brief fluctuations of a relatively thin fiber sliver, the above-described error has, however, aggravating consequences. This would be the case where the scanning roller would be used to measure at the output of a pre-spinning machine. EP 176 661 describes a possible application variant for the utilization of measuring signals obtained at the output of a process to control pre-spinning machines. However, no solution is offered for the avoidance of errors in the measuring signals produced by circularity error and/or eccentricity.
Brief fluctuations of the fiber sliver thickness occur within a range of fiber sliver lengths that is shorter than the circumference of a scanning roller. From this it clearly appears that periodically occurring errors due to circularity error and/or eccentricity of the scanning roller may have an enormous effect on the measuring signal.
This error influence increases as the thickness of the fiber sliver to be measured decreases. This explains why the measuring of tolerances and/or eccentricity becomes relevant beyond a certain ratio of the mean distance of the two scanning rollers.
According to the present state of the art, these errors can only be reduced by subjecting the scanning rollers to a costly manufacturing and testing process. To further reduce the known errors even slightly would lead to disproportionate expenditures. For this reason scanning rollers are not used at the output of pre-spinning machines to measure brief fluctuations of the fiber sliver thickness.